Box section - will it warp?

[Davek0974]

I am planning up the build of a new CNC plasma table, this time I'm using 50x50x2mm and 80x50x2mm box.

I need to join a strip of 3x30mm along the centre of the wide face of the 80x50 side frames, the length will be 1500mm

Was thinking of drilling and plug welding it after positioning and clamping it - do you think it will pull or warp the box section or not????

Thanks

 

[J.B. Welding]

Welding a strip on one side of a box section will always distort it. There is a term-'permissible deflection', that means how much bending can you stand! 80x50 is a good size of box, I'd say over 1.5m you will hardly notice it.
Not sure how you are welding the 30x3, flat or on edge?

 

[Davek0974]

strip needs to be flat-on, it supports the linear rail track from downward force and also spaces out the rack for the motion drive, keeping it parallel to the rail.

I was thinking an 8mm plug every 100-150mm ??

 

[J.B. Welding]

Well like I said there is bound to be a little distortion, could be a mm or 1.5 ctr to ends at a guess. Welding a strip to the other side may balance it up but if you want perfection could you screw it on with countersunk self tappers?

 

[cianh91]

That small a welds ur distortion will be minimal use heavier wall box and you will have very little

 

[Maker]

Bolt it on of you're worried about distortion.

 

[Richard.]

Welding a strip on one side of a box section will always distort it. There is a term-'permissible deflection', that means how much bending can you stand! 80x50 is a good size of box, I'd say over 1.5m you will hardly notice it.
Not sure how you are welding the 30x3, flat or on edge?

I've not heard that term in quite a while. Nice.
To the op. JB is right welding along the length will distort it but depending on the measures you take will control how much.
Clamp it down to a solid flat surface and leave it there till it's completely cool after welding. Do the minimum amount of welding needed and spread them out to spread the heat. If it's just a track to run something down then it's under no load so a few short stitches should do. Sounds crazy I know but turn the welding machine up nice and high and hit it with a fast progressive travel rate. Alot of people think turning power and wire speed down and slowly putting in a light run In will reduce heat input and distortion. Not the case at all. Hit it high for a short a time as possible is the answer. You've picked the best process. Tig would buckle it because it's too slow

 

[Davek0974]

Thanks, but....

It seems some did not read the OP 100% - I am talking of PLUG WELDING every 100-150mm NOT seam welding etc, I know for a fact that seam welding is not only pointless here but would also wreck the beam with warp.

But plug welds??????

 

[Gareth J]

I think the distortion will be barely measurable. 15 plug welds spread along the length of the very centre of the box. Such a small proportion of the face of the box will hot let alone molten.

Never done a directly comparable thing though so for the extra hassle, I'd just go with the suggestions above and tap the box for countersunk screws if yer not confident

 

[Kent]

I have my doubts about plug welds on such sections actually being strong enough, let alone the distortion effect. There must be better ways to achieve what you want, is it a case of you having these materials in stock? As said upping the wall thickness will help distortion but that than creates more issues with the plug welds which as always seems to be lack of fusion.

As always lots of tacks and back stepping or switching direction of weld sequence, clamping in fixtures can all be considered.

 

[Davek0974]

Yes, material is fixed at 50x80x2mm

I am backing the mounting screws for the linear rails and rack with strips of 5x20 to give the screws plenty of bite.

This strip will be taking the downward forces on the rail so needs to be reasonably firm, self tapping screws are out because of this.

If it does warp I could maybe press it out with the hydraulic press???

I think 1mm or less is probably acceptable on a 1500x1500mm box frame, I'm welding pads into the ends and bolt-assembling the frame to stop twist from welding the corners.

 

[Kent]

1mm on a 1500x1500 frame max tolerance is tough to obtain (not impossible but tough). Pressing it out? doubtful I fear. Its hard to advise really without a clear plan of everything, there has to be a better way of doing this but if I had 1mm max to play with on any frame I should be welding it clamped in a fixture for starters. you could already have 1mm run out before you touch it with a torch

 

[Davek0974]

Hmm, =maybe i was being a little tight there with 1mm

This is all self-design stuff, I have never used linear guide rails (HiWin) before, have no idea how a frame with a rail on each side that are joined by a rigid beam on support plates will work and how much tolerance there is.

All I do know is that many many cnc tables have been built this way and it does not look too difficult.

I was going to weld the frame solid but was advised not to and I am now thinking of welding in plates to the cross-beams and bolting the sides on, so I can shim for parallel etc.


An unscientific test - I took a 450mm scrap of the 80x50 box, a strip of 15x3 and plugged them together with an excessive amount of plugs, about every 40mm, 8mm dia. No effort was made to cool the beam between welds, I staggered end-to-end though, used a split clamp to hold it.

After welding it was too hot to hold. I checked for bow before I started on the other side using a straight edge and again after - no change was visible.

 

[J.B. Welding]

The structure of this 'flag' was made from vertical 50x25x3 box, with 30x3 welded on edge on the narrow side. This provided a fixing point for the vitreous enamelled panels. I ground a wee bit off the edge of the 30x3(1 inch, just a rub with a flap disk) every 10-12" to permit a weld to be entirely 'under' the 30x3, instead of welding the sides.
Anyway the longest one, about 1.5m, pulled maybe 3mm then more after galvanising. I had them well clamped to a heavy beam during construction and used a hot fast weld.. Hope that helps!
If any errors in this piece your making will be transmitted to the straight cuts on your machine I would weld it as little as possible as any curves will eventually drive you nuts!!
Just saw your last post....a test piece of 450mm might be a bit stingy...but you are using a bigger section than I did in this application so you may get away with it?
Guess when you weld up a big one you will know for sure-don't forget to report back

 

[Davek0974]

Nice,

The going way seems to be "as straight as you can build it", plasma is not super precise and you would not normally be cutting great big continuous straight edges - more usually lots of smaller parts from a bigger sheet. Some of the builds I look at are 10'x8' or bigger - the chances of perfection at that scale are slim i think.

You can't machine the sides of the beam - any material removed just releases more stress and the warp gets worse. I do not want to use super heavy section stuff - it's not needed and also comes with seriously large radii on the corners which means going to a bigger section just to get the real-estate back on the face.

I can't shim under the linear rail as it would need dozens of different thickness shims to shim a bow out.

The box I have seems pretty good but I guess the only way is to suck-it-and-see.

 

[mike os]

i suspect you are making it harder than it needs to be, have a visit to my cnc uk

 

[Davek0974]

Already there

I live on various forums - MyCncUK, Plasma Spider, Practical Machinist, this one and the CandCNC support one.

The MyCncUK one can be a bit quiet most of the time, build threads are few and far between, unless you have a specific one in mind??

 

[mike os]

not really.... for a plasma i am not sure the additional support is needed unless you cant drill the bolt holes accurately, in which case i would probably revert to using kickers rather than a welded strip... which wont be that straight before you start.

 

[Davek0974]

I can likely leave it out, the idea was to provide vertical support for the rail as well as space the rack parallel below it.

I could probably align the rail by making a jig or using a taut line as others have done and then spacing the rack by mounting an indicator on a truck and running it along the rail. The strip just sounded easier and just as effective.

 

[mike os]

For what it's doing I would d&t then... no distortion that way